WASHINGTON -- Al Gore -- star of an Oscar-winning movie, former U.S. vice president and the object of 2008 presidential speculation -- Wednesday took his crusade against global warming to Capitol Hill.

Glad-handing like the lifelong politician he was until losing the 2000 presidential race to George W. Bush, Gore called his return to Congress "an emotional occasion."

But he did not mince words on what he termed the current climate crisis: "Our world faces a true planetary emergency."

Before a joint House panel dealing with energy, air quality and the environment and the Senate Environment and Public Works committee, Gore stressed the need for quick action.

Under often contentious but consistently civil questioning at both hearings, Gore discussed the risks of sea level rise, stronger storms, more wildfires and other ills associated with global climate change, and urged an immediate freeze on U.S. carbon dioxide emissions.

After that, he said, the United States should begin a program of sharp reductions in carbon emissions "to reach at least 90 percent reductions by 2050." He also proposed a tax on carbon emissions.

Gore, a Democrat who represented Tennessee in Congress before serving as vice president under President Bill Clinton, had enough star power to pack a large hearing room and require three overflow rooms -- two for the public and one for media.

He has been nominated for a Nobel Peace Prize and has prompted intense curiosity in Washington about whether another presidential bid is in prospect. So far, he has said no but has not categorically ruled it out.

CARBON-NEUTRAL GORE?

Sen. James Inhofe, an Oklahoma Republican and Washington's most vocal skeptic about the human causes of global warming, pressed Gore to commit to cutting his personal home energy consumption to no more than what the average American household consumes -- without paying for carbon offsets, which Inhofe dismissed as "gimmicks used by the wealthy."

Gore demurred, but later said, "We live a carbon-neutral life, senator, and both of my businesses are carbon-neutral. We buy green energy, we do not contribute to the problem that I am joining with others to solve."

Living a carbon-neutral life means calculating how much climate-damaging carbon you emit, cutting emissions where possible and balancing the rest by buying so-called carbon offsets, such as shares in windmills or by planting trees.

Gore has lately faced public questions about his personal "carbon footprint," especially at his home in Tennessee. An aide noted that Gore and his wife Tipper drove to Wednesday's hearing in a black hybrid vehicle.

At the House hearing, he was flanked by cardboard boxes that he said contained some 516,000 letters calling for congressional action to stop global warming.

"This problem is burning a hole at the top of the world in the ice cover that is one of the principle ways our planet cools itself," Gore said. "If it goes, it won't come back on any timescale relevant to the human species."

Rep. Joe Barton, a Texas Republican, questioned the scientific basis for global warming as presented in "An Inconvenient Truth," the Academy Award-winning documentary that featured Gore.

Barton said a carbon tax would "harm the American family" adding, "A Kyoto-style cap and trade system will mainly increase the cost of electricity."

As for Gore's proposed freeze on carbon emissions, Barton said it would mean "no new industry, no new people and no new cars."

Other legislators, including former House Speaker Dennis Hastert, an Illinois Republican, said they agreed with much of what Gore said about climate change and its effects, but questioned the economics of tackling the problem and wondered whether any U.S. measures would put it at a competitive disadvantage with countries like China and India.